Improve API for sending binary

[johnhferland]

To send a slice of bytes across the wire we need to convert the bytes to a func () io.ReadCloser, which is pretty cumbersome. I would like to propose we expose an API where we can simply provide []byte and conjure-go will do the rest.
Before:

c.PutEndpoint(ctx, func() io.ReadCloser { return ioutil.NopCloser(bytes.NewReader(bodyBytes)) })

After:

c.PutEndpoint(ctx, bodyBytes)